Displays as, for example, liquid crystal displays (LCD) require a DC-free addressing voltage in order to prevent decomposition of the display. The LCD basically operates based on the root mean square (RMS) voltage provided by an AC voltage drop across a pixel. FIG. 1 shows a simplified circuit diagram of a single pixel of an LCD. VDATA is the addressing voltage for driving the pixel. An adjustable voltage buffer provides a common reference voltage VCOMOUT for the back plane of the LCD, which eliminates the DC portion of the voltage VPIX across the pixel and ensures that only an AC current flows through the pixel.
FIG. 2 shows a graph of a curve representing the required temperature dependency of the common reference voltage VCOMOUT in response to a temperature change of the LCD. VCOMOUT may remain constant up to a threshold value TTH of the display temperature. If the threshold temperature TTH is exceeded, the common reference voltage VCOMOUT should be increased with a steep slope SS. After this first steep increase, the ascending slope of the common reference voltage VCOMOUT decreases slowly. For example, for a temperature below the threshold temperature TTH of 40° C., the common reference voltage VCOMOUT may remain at 5.350 V. If the threshold temperature TTH of 40° C. is reached, an initial voltage step of about 0.03 to 0.04 V may be performed. Between 40° C. and 150° C. the common reference voltage is to be increased almost linearly until about 90° C. and after that with a decreasing slope until 5.75 V.
In order to comply with the diagram shown in FIG. 2, the circuit according to the prior art shown in FIG. 3 is used. A target voltage at node INPOS for the common reference voltage VCOMOUT is derived by a voltage divider R1, R2 from the supply voltage AVDD. Node INPOS is the positive input of the amplifier AMP, which may be configured as a voltage follower. The output of the amplifier AMP provides the common reference voltage VCOMOUT. A controlled current source CCS1 is also coupled to the positive input of the amplifier. CCS1 serves to adjust the common reference voltage VCOMOUT in accordance with a digital input signal DIN. The digital input signal DIN is a control signal which represents a current temperature of the LCD to which the common reference voltage VCOMOUT is supplied. Memory MEM contains a look-up table of digital values necessary to set the controlled current source CCS1 so as to provide the required output voltage VCOMOUT in response to a display temperature. Any type of LCD may require a different look-up table. The digital to analog converter DAC is adapted to provide an analog output signal based on the digital input signal DIN and the data received from the memory MEM. The output signal of the DAC controls the controlled current source CCS1 as a function of the temperature behavior of the LCD. However, the circuit shown in FIG. 3 is rather complex and requires a large (in terms of chip area of an integrated circuit) memory MEM for storing data values representing the relationship between VCOMOUT and the temperature of the LCD.